But with beauty customers more educated than ever, attention has turned towards the humble face cleanser. To parse all the products and figure out the best solution for you, you will need the dermis of an armadillo.

Good thing we have done some of the work for you.

THE IDEAL CLEANSER

Do not get distracted by slick marketing campaigns.

Dr Barbara Sturm, an aesthetic medical doctor in Germany who has a namesake skincare line, said the function of a daily cleanser should be straightforward.

It is "to remove dead skin cells, oil, dirt and other pollutants from the skin, unclog pores, prevent skin conditions such as acne, and prepare the skin for the next step in your skincare regimen".

In reality, though, it is a formulation challenge: A good cleanser must whisk away the bad stuff, yet magically leave your skin microbiome - micro-organisms that naturally live on your epidermis - relatively unscathed.

"Cleansing is really a delicate balance between hygiene and barrier damage," said Skinfix founder Amy Gordinier-Regan.

WHAT'S THE DEAL WITH PH?

The idea is that if the cleanser mimics the skin's naturally acidic pH (5.5), it will be more gentle on your skin's acid mantle - the protective, slightly acidic layer made up of natural oils, dead skin cells and sweat.

The acid mantle is what maintains skin health and staves off bacterial infections, said dermatologist Dendy Engelman.

Unfortunately, many traditional cleanser options, like soap and sulphate-based formulas (those that lather up easily), can skew high on the pH scale - soap is generally between nine and 10 - and strip down the acid mantle.

The problem is that "the definition of 'clean' can be subjective", Dr Engelman said.

"If, after washing, your skin feels tight, dry or inflamed, that means the cleanser or cleansing mechanism was too strong, because, in addition to cleansing, the surfactant has broken down a significant number of bonds in your skin that form your skin barrier."

That "squeaky clean" sensation many of us strive for is, she said, an unhealthy state for our skin to be in.

OIL VERSUS WATER

Cleansers fall into two categories: oil-based and water-based.

"Oil-based cleansers can do a wonderful job of maintaining the skin barrier," Ms Gordinier-Regan said. Oils also help break down make-up.

But, she said, "an oil-based cleanser will always leave behind some residue, so you want to make sure the oils in the cleanser are not clogging your pores".

If your skin is oily or prone to breakouts, Dr Sturm suggests to look for a water-based gel or foam cleanser.

The foaming aspect need not rely on sulphates. There are gentler surfactants available like decyl glucoside, which is often found in baby shampoos. But even if the product is gentle, she said, "it is important to quickly apply moisturiser to avoid moisture loss through osmosis".

You will want to avoid wipes when possible, though. While better than nothing, they are not a substitute for properly washing your face.

"And a wipe will leave ingredients behind that may irritate the skin," said Ms Hina Choudhary from the SkinCeuticals global scientific communications and medical relations team.

WHAT'S WITH TRIPLE CLEANSING?

Double and triple cleansing are ideas that sprang from the South Korean and Japanese beauty crazes of recent years.

The traditional K-beauty scenario involves using an oil-based cleanser to break down make-up.

And because some make-up, especially waterproof and long-wear formulas, is oil-based, it breaks down best with oil.

Then, because the oil cleanser leaves a residue, which is now mixed with the dirt and make-up, you follow it with "a traditional water-based foaming cleanser, which removes the oils and butters that the balms or oils leave on skin", said Ms Tiffany Masterson, the founder of skincare brand Drunk Elephant.

Problems pop up when you start washing with two water-based cleansers, which can result in overcleansing.

If you have dry skin and want some oil residue, Ms Masterson offers this hack: Use a water-based cleanser to get the grime off, then use the oil cleanser.

WASH JUST ONCE A DAY?

If you subscribe to Ms Masterson's theory on skin barrier maintenance, you should not be washing your face morning and night.

"If you have a good skincare routine, you should cleanse at night to get the grime and make-up off," she said. "Then add your serums and moisturisers, and while you're sleeping, you're nourishing your acid mantle. You don't want to wash that off in the morning."

Many people who have sensitive skin may simply be overwashing. "The industry is trying to sell as much as possible," Dr Sturm said.

Everyone should wash once a day, she said, and twice only if your skin tolerates it well.

TO EXFOLIATE OR NOT?

Be wary of cleansers loaded with acids, Ms Masterson said. "It's completely gimmicky to add all those acids, because cleansers are a rinse-off product, and you'd want your glycolic acid, for example, to have the chance to penetrate."

Dr Sturm takes an even more conservative approach, noting the abuse of exfoliators. You should be exfoliating only one or two times a week no matter the form, she said.

Yet with exfoliating acids - glycolic, lactic, salicylic and more - in so many formulations now, you can easily over-exfoliate without meaning to. Dr Engelman advises reading the ingredient lists closely.

"If you look at K-beauty or French beauty regimens, you'll notice that exfoliating is only one step, if any," she said.

"The belief is that if you give your skin everything it needs to perform optimally, you won't have to help it exfoliate itself. The truth is our skin naturally exfoliates itself through programmed cell turnover."

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